The present invention relates to coherent optical transmission and, more particularly, to an optical transmission format which facilitates switching and crossconnect processings within a communication system.
According to the conventional intensity modulation optical transmission system, the wavelength of a beam does not have an important effect. For example, a change of a wavelength during a transmission period has caused no problem so long as the change is within the range of wavelength reaching a sensitivity range of a receiver. However, according to the coherent optical transmission method, a change of a wavelength in transmission brings about a serious interference in performing normal communication, such as interchannel crosstalks or an erroneous selection of a channel, because optical signals of many wavelengths are multiplexed and transmitted in the coherent optical transmission system. Therefore, unnecessary change of a wavelength should be avoided when a switching processing is carried out in a transmission system. In other words, there may occur a problem unless a wavelength is correctly converted when it becomes necessary to rewrite control information following a switching processing. In order to solve the above problem, it is necessary to arrange such that all the information essential for a switching operation such as attributes of the signal, destinations, etc. is being carried as a sub-signal and the switching operation is carried on based on this sub-signal, with a main signal kept unchanged.
There has been no case that a sub-signal is superposed on a main signal to achieve the above-described purpose in the conventional coherent transmission method. In a system using an intensity modulation, a sub-signal superposing method has been proposed, such as the one described in "Design of Lithium Niobate Based Photonic Switching Systems" by W. A. Payne, et al, IEEE Comm. Magazine, vol. 25, May, 1987, pp. 37-41.
If the above method of using the same modulation system for both the main signal and the sub-signal is directly applied to the coherent transmission method, it becomes necessary to carry out coherent decoding each time when a processing is carried out. Since fine tuning is necessary in stabilizing the wavelength of a receiving signal, coherent decoding will require a complicated structure of the receiver. There is also a risk that a wavelength of the receiving signal is deviated from a correct range, making it difficult to materialize a practical application of the system.